The intermetallic compound triniobium tin, Nb.sub.3 Sn, is a type-II metallic superconductor of interest because it has high values of superconducting critical current density in high magnetic fields. Historically, Nb.sub.3 Sn has been formed using a number of different processes. These include: 1) condensation from the vapor phase; 2) crystallization from the liquid phase; 3) diffusion between one solid phase and one liquid phase; and 4) diffusion between two solid phases.
Methods for forming triniobium tin superconductors by liquid-solid diffusion require multiple steps. The general method is well known and a description of this method is given below.
Generally, the first step in forming triniobium tin superconductor is to clean the niobium or niobium based substrate. This is done with a cleaning solution or etchant, such as a mixture of nitric acid, hydrochloric acid, and water. Diluted hydrofluoric acid is also sometimes used for cleaning the substrate. After the substrate is cleaned, oxygen may be added to the surface of the substrate by anodizing the surface electrolytically.
The next three steps involve high temperature heat treatments. The first anneal, as taught by Caslaw in British patent 1,342,726, is used to introduce a desired oxygen content into the niobium substrate. This is accomplished by passing the substrate through a furnace at about 950.degree. C. for about 30 seconds in an atmosphere containing argon and oxygen. However, if the substrate has been previously anodized to form an oxide layer on the surface of the substrate, then the preheat is called a decomposition anneal whereby the substrate is annealed so that the oxide layer diffuses into the body of the substrate. This would be conducted in an oxygen free environment.
After the preheat, the substrate is dipped in a tin or tin alloy bath, which supplies the tin for the triniobium tin reaction. The operating temperature for the tin bath is between 900.degree.-1000.degree. C.
The tin coating from the bath has a limiting thickness due to the amount of tin needed to further react with the niobium. This thickness is about two to thirty micrometers of tin. The tin thickness is inversely proportional to the viscosity of the tin bath and the temperature of the tin bath, and proportional to the foil speed. Thus, the tin thickness places a lower limit on the substrate speed in the tin dip, which, as an example, is about thirteen centimeters per second for a tin bath at 900.degree.-1000.degree.0 C.
Subsequently, after the tin coating is applied, the niobium substrate is cooled before being treated with a reaction anneal to react the tin coating and the niobium base metal. During this final anneal, a layer of superconducting triniobium tin is formed on both sides of the niobium substrate. In the reaction anneal, the foil is moved through a furnace at about 1050.degree. C. and the speed of the foil depends on the anneal time and the height of the furnace. For instance, in a five meter furnace for about two hundred seconds at 1050.degree. C., the foil speed is about 2.5 centimeters per second.
The above-mentioned steps, the preheat step, the tin dip, and the reaction anneal, are generally performed separately due to space limitations in manufacturing areas and variations in the substrate speed through the different steps.
Thus, there is a need to have an efficient process for making triniobium tin that combines the three high temperature steps into one, continuous cycle within a chamber having uniform atmosphere conditions.
There is also a need to utilize one substrate speed through the different high temperature steps. Further, there is a need for a triniobium tin method that has a low tin bath temperature yielding slow foil speeds and thick tin deposits, while the reaction anneal step operates at a higher temperature with shorter furnace lengths.